you should not have to do this, so up front I agree that they need work. but..

like anything in this game, you can find creative uses for them. I have a border forest/meadows base that is lively with dwarf, troll, fuling, boar, deer, neck, and the occasional lost skeleton.
Ive set up 3 of the ballistas in protected black marble housings and load them with about 5 shots each. After a busy night, all 3 are shot out empty and I can just loot the next morning and reload them. Pretty good addition to the existing warzone though the fulings just kill everything so its not much better than just cleaning those out after they have had their fun.

My complaint is that they miss too much, so each kill takes 2-3 shots or more if the enemy starts running in panic.

you know they shoot at you. the range is short, the angle is limited, so you can arrange to not cover an area you want to walk in, or you can wait on them to empty before going out there, or you can just avoid being shot by not standing still and moving lateral to its fire.